The present invention relates to probe stations, commonly known as package or wafer probers, used manually, semi-automatically, or fully automatically to test electrical devices such as semiconductor wafers.
Existing probe stations are capable of performing both low-current and high frequency measurements in an electronically quiet environment. The environment may be provided by, for example, incorporating one or more guard and electromagnetic interference (EMI) shield structures within an environmental enclosure. Guard and EMI shield structures are well known and discussed extensively in technical literature. See, for example, an article by William Knauer entitled “Fixturing for Low Current/Low Voltage Parametric Testing” appearing in Evaluation Engineering, November, 1990, pages 150-153. Examples of existing probe stations that provide such guard and EMI shield structures can be found in commonly owned U.S. Pat. Nos. 5,434,512; and 5,266,889 which are hereby incorporated by reference.
Probe stations deliver a test signal to an electrical device, such as a semiconductor wafer, whose characteristics are to be measured. Test conditions are desirably controlled and substantially free of electromagnetic interference, though not necessarily, that may emanate from test instrumentation or other nearby electrical equipment, or that may result from spurious air currents or the like. To provide a controlled and substantially noise-free test environment, existing probe stations that incorporate guard structures will usually at least partially surround the test signal path with a guard signal that closely approximates the test signal, thus inhibiting electromagnetic current leakage from the test signal path to its immediately surrounding environment. Similarly, EMI shield structures may provide a shield signal to the environmental enclosure surrounding much of the perimeter of the probing environment. The environmental enclosure may typically be connected to shield, earth ground, instrumentation ground, or some other desired potential.
To provide test, guard, and shield signals to the probe station, existing probe stations often include a multistage chuck upon which the electrical device rests while being tested. The top stage of the chuck, which supports the electrical device, typically comprises a solid, electrically conductive metal plate through which the test signal may be routed. A middle stage and a bottom stage of the chuck similarly comprise solid electrically conductive plates through which a guard signal and a shield signal may be routed, respectively. In this fashion, an electrical device resting on such a multistage chuck may be both guarded and shielded from below. Similarly, single stage and dual stage chucks, and chucks with substantial openings centrally defined therein are likewise frequently employed.
Further reduction in interference can be obtained by locating a suspended conductive plate over the electrical device which is typically electrically insulated from the test signal path and connected to the guard signal. The suspended plate defines a central opening so that the probe assembly may make electrical contact with the electrical device. In this fashion, the electrical device can be guarded from both below and above by signals closely approximating that delivered to the electrical device.
Though such a probe station is effective in performing low-current testing and high frequency testing of electrical devices, the aforementioned existing probe stations unfortunately often exhibit significant inductance to high current measurements, and particularly when testing using pulsed signals. The high inductance tends to resist fast changes in the current levels, and results in higher than desirable voltage and current levels.
What is desired, therefore, is a probe station that is suitable for performing high current and/or pulsed tests.